Monday, September 5, 2011

RT / Radiographic Procedure

1.0    SCOPE

This procedure describes the minimum requirement of Radiographic examination of welds using Iridium -192 (Gamma Ray). This procedure has specific reference to the

2.0    REFERENCES


2. 1    ASME Sec V            -  Non Destructive Examination
2. 2    ASNT-SNT-TC1A      - Recommended Practice for Non-Destructive
Testing Personnel 
2. 3    ASME Sec VIII           - Pressure Vessels
2. 4    ASME B31.1               - Power Piping
2. 5    ASME B31.3               - Chemical Plant and Petroleum Refinery Piping
2. 6    ASME B31.4               - Liquid Transportation Systems for Hydrocarbons
2. 7    ASME B31.8               - Gas Transmission and Distribution Piping Systems
2. 8    API 650                       - Welded Steel Tanks for Oil Storage
2. 9    API 1104                     - Welding of Pipe lines & related facilities
2. 10  AWS D1.1                   - Structural steel welding code



3.0    DEFINITIONS


3.1     ASNT                    -       American Society for Non-destructive Testing
3. 2    ASME                   -        American Standard for Mechanical Engineers
3. 3    API                       -         American Petroleum Institute
3. 4    AWS                     -         American Welding Society
   



4.0    PROCEDURE

4.1       PERSONNEL QUALIFICATION

4.1.1  All personnel will be qualified in accordance with ASNT SNT-TC I-A
Level -II.
4.1.2  Film interpreter shall have yearly eye examination and certificate
shall be available for verification.
4.1.3  Complete qualification records for each radiographer shall be
submitted for contractor approval prior to the beginning of any radiographic work. 

4.2       CONSUMABLE

4.2.1 Chemicals
Processing chemicals shall be prepared as per manufacturer's recommendation.
4.2.2 Film
Kodak,Agfa or Fuji films shall be used for radiography.

Film Type                                Material Thickness

Class I                    -                t < 0.5”
Class II                   -                t ³ 0.5” to £ 1.0”
Class III                  -                t > 1.0”
        
4.2.3  Lead Intensifying Screens

Intensifying screen shall be 0.125mm thick (lead) for both front and back. Additional external shielding may be required to avoid any back scattered radiation. All screens shall be handled carefully to avoid dents, scratches, dirt etc. on surfaces. All damaged screens shall be discarded.
Chemically coated, fluorescent or fluormetallic intensifying screens shall note be used unless specifically approved by the client.

4.3       NUMBER OF EXPOSURES

An adequate number of exposures shall be made to demonstrate that the required coverage has been made. At the end of films the penetrated thickness shall not be more than 10% of thickness of weld.
4.4    BACK SCATTER RADIATION
4.4.1       A lead symbol “B” with minimum dimensions of 1/2 in. in height and 1/16 in. in thickness, shall be attached to the back of film holder to determine if backscatter radiation is exposing the film.

4.4.2       If a light  image of the “B” appears on a darker background of the radiograph, protection from backscatter is insufficient and the radiograph shall be considered unacceptable.

4.5       IMAGE QUALITY INDICATOR(IQI)

4.5.1    IQI Shall be used for the varification of radiographic sensitivity. Radiography shall be executed with a technique of sufficient sensitivity to display the IQI image and specified wire number listed in Table – 1.

4.5.2    Wire number shall be based on nominal single wall thickness plus appropriate weld reinforcement pad in the case of DWSI or SWSI. The thickness laying between source and film shall be considered in the case of DWDI.

4.5.3    Backing rings or strips and root penetration are not to be considered as part of the weld or reinforcement thickness in selection of the IQI.

4.5.4    IQI shall be placed so that at least one is visible on each radiograph. Where a panoramic exposure is taken then a minimum of three equally spaced IQI shall be used or other as specified by the reference code. The IQI is placed across the weld seam so that the wires are perpendicular to the weld axis.
    
4.5.5    The IQI shall be placed on the source side of the part being tested, except for the condition described in 4.5.6.

4.5.6    Where inaccessibility prevents hand placing the IQI on the source side, the IQI shall be placed on the film side of the part and a lead letter “F” of dimensions at least  the same as the IQI identification shall be placed adjacent to the IQI identification.

4.5.7    The material of the IQI shall be of similar radiographic density to that of the material under examination, i.e. use steel for steel, aluminum for aluminum, etc.,
4.6       SENSITIVITY

Radiographic sensitivity is calculated using the following formula and all the radiograph shall have a minimum 2% sensitivity.

% Sensitivity  = Diameter of thinnest wire visible x100
     Wall thickness + reinforcement

4.7       INHERENT UNSHARPNESS

The inherent unsharpness (Ug) shall be calculated using the following formula.

Ug.=F.OFD/SOD

Where Ug = Geometric unsharpness(mm)
F = Focal spot /Sorce size in mm, the maximum effective dimension of the radiating focal spot / source in the plane normal to the line of the SOD
OFD = Object to film distance (mm),distance from the source side of the object being radiographed to the film.
SOD = Source to Object distance (mm),distance from the source of radiation to weld or other object being radiographed.

Material Thickness( inch)                   Maximum Ug (inch)
Under 2                                               0.020
2 through 3                                          0.030
Over 3 through 4                                  0.040
Greater than 4                                      0.070

5.0       RADIOGRAPHIC TECHNIQUES

Standard techniques (Attached) shall be utilized where possible.

Technique                                           Pipe Diameter(NPS)

Single Wall Single Image(SWSI)         Over 6"
                    Double Wall Double Image( DWDI)     £ 3”
                    Double Wall Single Image (DWSI)      ³ 3”


5.1     Film Identification

Each radiograph shall be identified through the use of lead markers(numbers and letters). The identification shall be as specified by the client but should consist of :-

1. Project Identification
2. Component, Vessel or piping identifications
3. Seams or welds identification
4. R for repair, R2, R3 etc. if more than one repair.
5. C for new weld following complete cut out
                              6. Date of radiography
                              7. Welders identification

5.2     Surface Preparation

Welds which are to be examined radiographically shall be prepared by the contractor to the extent necessary to ensure that, in the resulting radiographic image any irregularities will not mask or be confused with the image of any unacceptable discontinuity on the radiograph.

5.3     MARKING

5.3.1        For single viewing (SV) techniques a starting point shall be selected for each  weld being radiographed.  The starting point shall be accurately marked on pipe showing the datum and direction of the marker belt.  The belt shall start with zero at the arrow and encircle the pipe.  The arrow direction and location shall be consistent on any given job.

5.3.2        For double viewing (DV) techniques, markers shall usually be as single  marker mounted centrally above the area of interest.  The first exposure marker designated “A” the second “B” and so on if required.  The location of the marker shall be marked on the object.

5.3.3        Where the weld is ground smooth with the adjacent base metal and it is not evident where the edges of the weld begin, marker arrows or “V” shall be supplied placed at the ends of the intended area for interpretation.  Markers shall be placed at the end of the intended area for Interpretation.
5.3.4  If mechanical means of marking the datum and direction are to be used, this   shall be supplied by the Contractor unless otherwise specified.  If metal stamping is not permitted in a particular situation, vibroetching, electroetching or other techniques may be used as specifically approved by the Client.
                        
5.4       FILM PROCESSING

5.4.1  Film shall be processed to comply with the film
         manufacturer’s  recommendations as to required chemicals,
         processing techniques and times.

5.4.2  All radiographs shall be free from mechanical and chemical
          marks or other artifacts to the extent that they cannot mask
          or be confused with the image of any discontinuity in the area
          of interest of the object being radiographed.

5.4.3   Processing will be manual.

5.4.3  Development time shall be 3 to 5 minutes in developer
solution at 20 °C with constant agitation.

5.4.4  Following development the film shall be agitated for at least one minute  in acetic acid stop bath.

5.4.5  Films will be fixed for at least twice the clearing time with agitation for the first 20 seconds.

5.4.6  Washing will be for at least 15 minutes in running water.

5.4.7  Drying temperature shall not exceed 40 °C.

5.4.8  Particular care shall be taken on dilution, chemical life and replenishment as recommended by manufacturers.

5.5     FILM DENSITY

5.5.1  Radiographic density in area of interest shall be not less than
          1.8 and not greater than 3.6 unless adequate viewing and
          satisfactory interpretation of higher density films are
          permitted by the viewing equipment.

                   5.5.2   The weld metal film in the area of iterest shall not vary by
                              more than plus 30% or minus 15% of the density adjacent to
                              the IQI, excepting as described in the item no. 5.5.3.
                   
                   5.5.3    I f more than one IQI is used, one shall be representative of
 higher density  

5.6     FILM VIEWING

5.6.1   Equipment used to view radiographs for interpretation shall
           have a variable     light source sufficient for the essential designated IQI wire to be visible for the specified density range of 2.0 to 4.0 High intensity light sources shall have exhaust arrangements for cooling to prevent film damage.

5.6.2   Light from the viewer not transmitted through the film shall be masked when viewing the radiograph.

5.6.3   Calibrated densitometers or step wedge films shall be used to assure film density compliance

  6.0     Quality of Radiographs

All radiographs shall be free from mechanical, chemical or other blemishes to the extent that they do not mask and are not confused with the image of any discontinuity in the area of interest of the object being radiographed. Such blemishes include

a)       fogging
b)       processing defects such as streaks, watermarks, or chemical stains
c)       scratches, finger marks, crimps, dirtiness, static marks, smudges or tears
d)       false indications due to defective screens.



7.0       FILM INTERPRETATION

·        Radiographs shall be interpreted by Level II or Level III personnel
·        Acceptance level shall be as per relevant project code


8.0       SAFETY

·        Every personnel involved in radiography shall wear personal monitoring film badge.
·        The team leader shall have approval of Radiation Protection Dept. of Ministry of Public Health, Kuwait.
·        The radiation area (3mSv/hr limit) shall be cordoned off with rope and all non radiography personnel shall be evacuated from the area prior to start of work. Radiation warning signs shall be posted at sufficient locations along the rope and monitored by the technician.
·        Fire and safety regulations of clients shall be adhered to at all times.

9.0    ATTACHMENT
    
          Attachment A         -        Radiographic Techniques
          Attachment B         -        Radiographic Report Form
          Attachment C        -         IQI Selection Table
 

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